I was going to say exactly the same...Winbox closes suddenly If I press a menu button after saving session.
RouterOS 6.24, Win XP SP3.
Name of the session doesn't has any strange character, only alphabetical characters.
:local macaddress [/interface get ether1 mac-address]; :put $macaddress
chain=input action=accept protocol=tcp in-interface=ether2-WAN-2 dst-port=8291 log=no log-prefix=""